The Current Release and Under Development Redirects should be adjusted as soon as possible after the new release comes out. We should always have a Release Notes page for one unreleased version, to which Under Development should point (as of 4 Feb 06, that's still 0.19), and the Current Release link should point to whatever it actually out (0.18).
What's New will point the same place as Current Release for a reasonable period after a new release, and then will be pointed to the page to which Under Development points, when enough traffic has accumulated on that page from SVN commits to merit it.
The links on the Main page release box should point directly, since they have to be updated anyway. All 5 items should be changed at release time (which is based on the timestamp of Isaac's message on the mailing lists).